Translation:They were strong even when they were losing.
I think adding "being" to the translation does not change the meaning, - but is probably to complicate things more than necessary.
Imperfect is latin and means unfinished. Something that happened some time back but never, or at least not yet has been finished. As this verb form does not exist in English it is a bit tricky to translate but in most cases you can use "was/where +verb" (past progressive tense) or "used to +verb" to translate it.
If you want to negate it you can use "was/were not +verb" or "did not use to +verb" / "never used to +verb" or to put emphasis to it "used to not +verb"